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          字符串的新增方法
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      </script></div> <div class="theme-vdoing-content content__default"><h1 id="字符串的新增方法"><a href="#字符串的新增方法" class="header-anchor">#</a> 字符串的新增方法</h1> <p>本章介绍字符串对象的新增方法。</p> <h2 id="string-fromcodepoint"><a href="#string-fromcodepoint" class="header-anchor">#</a> String.fromCodePoint()</h2> <p>ES5 提供<code>String.fromCharCode()</code>方法，用于从 Unicode 码点返回对应字符，但是这个方法不能识别码点大于<code>0xFFFF</code>的字符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>String<span class="token punctuation">.</span><span class="token function">fromCharCode</span><span class="token punctuation">(</span><span class="token number">0x20BB7</span><span class="token punctuation">)</span>
<span class="token comment">// &quot;ஷ&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div> <p>上面代码中，<code>String.fromCharCode()</code>不能识别大于<code>0xFFFF</code>的码点，所以<code>0x20BB7</code>就发生了溢出，最高位<code>2</code>被舍弃了，最后返回码点<code>U+0BB7</code>对应的字符，而不是码点<code>U+20BB7</code>对应的字符。</p> <p>ES6 提供了<code>String.fromCodePoint()</code>方法，可以识别大于<code>0xFFFF</code>的字符，弥补了<code>String.fromCharCode()</code>方法的不足。在作用上，正好与下面的<code>codePointAt()</code>方法相反。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>String<span class="token punctuation">.</span><span class="token function">fromCodePoint</span><span class="token punctuation">(</span><span class="token number">0x20BB7</span><span class="token punctuation">)</span>
<span class="token comment">// &quot;𠮷&quot;</span>
String<span class="token punctuation">.</span><span class="token function">fromCodePoint</span><span class="token punctuation">(</span><span class="token number">0x78</span><span class="token punctuation">,</span> <span class="token number">0x1f680</span><span class="token punctuation">,</span> <span class="token number">0x79</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">'x\uD83D\uDE80y'</span>
<span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，如果<code>String.fromCodePoint</code>方法有多个参数，则它们会被合并成一个字符串返回。</p> <p>注意，<code>fromCodePoint</code>方法定义在<code>String</code>对象上，而<code>codePointAt</code>方法定义在字符串的实例对象上。</p> <h2 id="string-raw"><a href="#string-raw" class="header-anchor">#</a> String.raw()</h2> <p>ES6 还为原生的 String 对象，提供了一个<code>raw()</code>方法。该方法返回一个斜杠都被转义（即斜杠前面再加一个斜杠）的字符串，往往用于模板字符串的处理方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>String<span class="token punctuation">.</span>raw<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Hi\n</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span><span class="token number">2</span><span class="token operator">+</span><span class="token number">3</span><span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">!</span><span class="token template-punctuation string">`</span></span>
<span class="token comment">// 实际返回 &quot;Hi\\n5!&quot;，显示的是转义后的结果 &quot;Hi\n5!&quot;</span>

String<span class="token punctuation">.</span>raw<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Hi\u000A!</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">;</span>
<span class="token comment">// 实际返回 &quot;Hi\\u000A!&quot;，显示的是转义后的结果 &quot;Hi\u000A!&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果原字符串的斜杠已经转义，那么<code>String.raw()</code>会进行再次转义。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>String<span class="token punctuation">.</span>raw<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Hi\\n</span><span class="token template-punctuation string">`</span></span>
<span class="token comment">// 返回 &quot;Hi\\\\n&quot;</span>

String<span class="token punctuation">.</span>raw<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">Hi\\n</span><span class="token template-punctuation string">`</span></span> <span class="token operator">===</span> <span class="token string">&quot;Hi\\\\n&quot;</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>String.raw()</code>方法可以作为处理模板字符串的基本方法，它会将所有变量替换，而且对斜杠进行转义，方便下一步作为字符串来使用。</p> <p><code>String.raw()</code>本质上是一个正常的函数，只是专用于模板字符串的标签函数。如果写成正常函数的形式，它的第一个参数，应该是一个具有<code>raw</code>属性的对象，且<code>raw</code>属性的值应该是一个数组，对应模板字符串解析后的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// `foo${1 + 2}bar`</span>
<span class="token comment">// 等同于</span>
String<span class="token punctuation">.</span><span class="token function">raw</span><span class="token punctuation">(</span><span class="token punctuation">{</span> raw<span class="token operator">:</span> <span class="token punctuation">[</span><span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">]</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1</span> <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;foo3bar&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>String.raw()</code>方法的第一个参数是一个对象，它的<code>raw</code>属性等同于原始的模板字符串解析后得到的数组。</p> <p>作为函数，<code>String.raw()</code>的代码实现基本如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>String<span class="token punctuation">.</span><span class="token function-variable function">raw</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">strings<span class="token punctuation">,</span> <span class="token operator">...</span>values</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> output <span class="token operator">=</span> <span class="token string">''</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> index<span class="token punctuation">;</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span>index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> index <span class="token operator">&lt;</span> values<span class="token punctuation">.</span>length<span class="token punctuation">;</span> index<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    output <span class="token operator">+=</span> strings<span class="token punctuation">.</span>raw<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">+</span> values<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  output <span class="token operator">+=</span> strings<span class="token punctuation">.</span>raw<span class="token punctuation">[</span>index<span class="token punctuation">]</span>
  <span class="token keyword">return</span> output<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h2 id="实例方法-codepointat"><a href="#实例方法-codepointat" class="header-anchor">#</a> 实例方法：codePointAt()</h2> <p>JavaScript 内部，字符以 UTF-16 的格式储存，每个字符固定为<code>2</code>个字节。对于那些需要<code>4</code>个字节储存的字符（Unicode 码点大于<code>0xFFFF</code>的字符），JavaScript 会认为它们是两个字符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> s <span class="token operator">=</span> <span class="token string">&quot;𠮷&quot;</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span>length <span class="token comment">// 2</span>
s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// ''</span>
s<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// ''</span>
s<span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 55362</span>
s<span class="token punctuation">.</span><span class="token function">charCodeAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 57271</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，汉字“𠮷”（注意，这个字不是“吉祥”的“吉”）的码点是<code>0x20BB7</code>，UTF-16 编码为<code>0xD842 0xDFB7</code>（十进制为<code>55362 57271</code>），需要<code>4</code>个字节储存。对于这种<code>4</code>个字节的字符，JavaScript 不能正确处理，字符串长度会误判为<code>2</code>，而且<code>charAt()</code>方法无法读取整个字符，<code>charCodeAt()</code>方法只能分别返回前两个字节和后两个字节的值。</p> <p>ES6 提供了<code>codePointAt()</code>方法，能够正确处理 4 个字节储存的字符，返回一个字符的码点。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'𠮷a'</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 134071</span>
s<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 57271</span>

s<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 97</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>codePointAt()</code>方法的参数，是字符在字符串中的位置（从 0 开始）。上面代码中，JavaScript 将“𠮷a”视为三个字符，codePointAt 方法在第一个字符上，正确地识别了“𠮷”，返回了它的十进制码点 134071（即十六进制的<code>20BB7</code>）。在第二个字符（即“𠮷”的后两个字节）和第三个字符“a”上，<code>codePointAt()</code>方法的结果与<code>charCodeAt()</code>方法相同。</p> <p>总之，<code>codePointAt()</code>方法会正确返回 32 位的 UTF-16 字符的码点。对于那些两个字节储存的常规字符，它的返回结果与<code>charCodeAt()</code>方法相同。</p> <p><code>codePointAt()</code>方法返回的是码点的十进制值，如果想要十六进制的值，可以使用<code>toString()</code>方法转换一下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'𠮷a'</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span> <span class="token comment">// &quot;20bb7&quot;</span>
s<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span> <span class="token comment">// &quot;61&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>你可能注意到了，<code>codePointAt()</code>方法的参数，仍然是不正确的。比如，上面代码中，字符<code>a</code>在字符串<code>s</code>的正确位置序号应该是 1，但是必须向<code>codePointAt()</code>方法传入 2。解决这个问题的一个办法是使用<code>for...of</code>循环，因为它会正确识别 32 位的 UTF-16 字符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'𠮷a'</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> ch <span class="token keyword">of</span> s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>ch<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 20bb7</span>
<span class="token comment">// 61</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>另一种方法也可以，使用扩展运算符（<code>...</code>）进行展开运算。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span><span class="token string">'𠮷a'</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// arr.length === 2</span>
arr<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>
  <span class="token parameter">ch</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>ch<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 20bb7</span>
<span class="token comment">// 61</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>codePointAt()</code>方法是测试一个字符由两个字节还是由四个字节组成的最简单方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">is32Bit</span><span class="token punctuation">(</span><span class="token parameter">c</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> c<span class="token punctuation">.</span><span class="token function">codePointAt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">0xFFFF</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">is32Bit</span><span class="token punctuation">(</span><span class="token string">&quot;𠮷&quot;</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
<span class="token function">is32Bit</span><span class="token punctuation">(</span><span class="token string">&quot;a&quot;</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h2 id="实例方法-normalize"><a href="#实例方法-normalize" class="header-anchor">#</a> 实例方法：normalize()</h2> <p>许多欧洲语言有语调符号和重音符号。为了表示它们，Unicode 提供了两种方法。一种是直接提供带重音符号的字符，比如<code>Ǒ</code>（\u01D1）。另一种是提供合成符号（combining character），即原字符与重音符号的合成，两个字符合成一个字符，比如<code>O</code>（\u004F）和<code>ˇ</code>（\u030C）合成<code>Ǒ</code>（\u004F\u030C）。</p> <p>这两种表示方法，在视觉和语义上都等价，但是 JavaScript 不能识别。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'\u01D1'</span><span class="token operator">===</span><span class="token string">'\u004F\u030C'</span> <span class="token comment">//false</span>

<span class="token string">'\u01D1'</span><span class="token punctuation">.</span>length <span class="token comment">// 1</span>
<span class="token string">'\u004F\u030C'</span><span class="token punctuation">.</span>length <span class="token comment">// 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码表示，JavaScript 将合成字符视为两个字符，导致两种表示方法不相等。</p> <p>ES6 提供字符串实例的<code>normalize()</code>方法，用来将字符的不同表示方法统一为同样的形式，这称为 Unicode 正规化。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'\u01D1'</span><span class="token punctuation">.</span><span class="token function">normalize</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">'\u004F\u030C'</span><span class="token punctuation">.</span><span class="token function">normalize</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>normalize</code>方法可以接受一个参数来指定<code>normalize</code>的方式，参数的四个可选值如下。</p> <ul><li><code>NFC</code>，默认参数，表示“标准等价合成”（Normalization Form Canonical Composition），返回多个简单字符的合成字符。所谓“标准等价”指的是视觉和语义上的等价。</li> <li><code>NFD</code>，表示“标准等价分解”（Normalization Form Canonical Decomposition），即在标准等价的前提下，返回合成字符分解的多个简单字符。</li> <li><code>NFKC</code>，表示“兼容等价合成”（Normalization Form Compatibility Composition），返回合成字符。所谓“兼容等价”指的是语义上存在等价，但视觉上不等价，比如“囍”和“喜喜”。（这只是用来举例，<code>normalize</code>方法不能识别中文。）</li> <li><code>NFKD</code>，表示“兼容等价分解”（Normalization Form Compatibility Decomposition），即在兼容等价的前提下，返回合成字符分解的多个简单字符。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'\u004F\u030C'</span><span class="token punctuation">.</span><span class="token function">normalize</span><span class="token punctuation">(</span><span class="token string">'NFC'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>length <span class="token comment">// 1</span>
<span class="token string">'\u004F\u030C'</span><span class="token punctuation">.</span><span class="token function">normalize</span><span class="token punctuation">(</span><span class="token string">'NFD'</span><span class="token punctuation">)</span><span class="token punctuation">.</span>length <span class="token comment">// 2</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码表示，<code>NFC</code>参数返回字符的合成形式，<code>NFD</code>参数返回字符的分解形式。</p> <p>不过，<code>normalize</code>方法目前不能识别三个或三个以上字符的合成。这种情况下，还是只能使用正则表达式，通过 Unicode 编号区间判断。</p> <h2 id="实例方法-includes-包含-startswith-头部包含-endswith-末尾包含"><a href="#实例方法-includes-包含-startswith-头部包含-endswith-末尾包含" class="header-anchor">#</a> 实例方法：includes() 包含, startsWith() 头部包含, endsWith() 末尾包含</h2> <p>传统上，JavaScript 只有<code>indexOf</code>方法，可以用来确定一个字符串是否包含在另一个字符串中。ES6 又提供了三种新方法。</p> <ul><li><strong>includes()</strong>：返回布尔值，表示<strong>是否找到了参数字符串</strong>。</li> <li><strong>startsWith()</strong>：返回布尔值，表示<strong>参数字符串是否在原字符串的头部</strong>。</li> <li><strong>endsWith()</strong>：返回布尔值，表示<strong>参数字符串是否在原字符串的尾部</strong>。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'Hello world!'</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span><span class="token function">startsWith</span><span class="token punctuation">(</span><span class="token string">'Hello'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
s<span class="token punctuation">.</span><span class="token function">endsWith</span><span class="token punctuation">(</span><span class="token string">'!'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
s<span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token string">'o'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>这三个方法都支持第二个参数，表示开始搜索的位置。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">'Hello world!'</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span><span class="token function">startsWith</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
s<span class="token punctuation">.</span><span class="token function">endsWith</span><span class="token punctuation">(</span><span class="token string">'Hello'</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
s<span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token string">'Hello'</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码表示，使用第二个参数<code>n</code>时，<code>endsWith</code>的行为与其他两个方法有所不同。它针对前<code>n</code>个字符，而其他两个方法针对从第<code>n</code>个位置直到字符串结束。</p> <h2 id="实例方法-repeat-重复"><a href="#实例方法-repeat-重复" class="header-anchor">#</a> 实例方法：repeat() 重复</h2> <p><code>repeat</code>方法返回一个新字符串，表示将原字符串重复<code>n</code>次。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// &quot;xxx&quot;</span>
<span class="token string">'hello'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// &quot;hellohello&quot;</span>
<span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>参数如果是小数，会被取整。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">2.9</span><span class="token punctuation">)</span> <span class="token comment">// &quot;nana&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果<code>repeat</code>的参数是负数或者<code>Infinity</code>，会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span>
<span class="token comment">// RangeError</span>
<span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token comment">// RangeError</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>但是，如果参数是 0 到-1 之间的小数，则等同于 0，这是因为会先进行取整运算。0 到-1 之间的小数，取整以后等于<code>-0</code>，<code>repeat</code>视同为 0。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">0.9</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>参数<code>NaN</code>等同于 0。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果<code>repeat</code>的参数是字符串，则会先转换成数字。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token string">'na'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;&quot;</span>
<span class="token string">'na'</span><span class="token punctuation">.</span><span class="token function">repeat</span><span class="token punctuation">(</span><span class="token string">'3'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;nanana&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="实例方法-padstart-补全头部-padend-补全末尾"><a href="#实例方法-padstart-补全头部-padend-补全末尾" class="header-anchor">#</a> 实例方法：padStart() 补全头部，padEnd() 补全末尾</h2> <p>ES2017 引入了字符串补全长度的功能。如果某个字符串不够指定长度，会在头部或尾部补全。<code>padStart()</code>用于<strong>头部补全</strong>，<code>padEnd()</code>用于<strong>尾部补全</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'ababx'</span>
<span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'abax'</span>

<span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padEnd</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'xabab'</span>
<span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padEnd</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'xaba'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>padStart()</code>和<code>padEnd()</code>一共接受两个参数，第一个参数是字符串补全生效的最大长度，第二个参数是用来补全的字符串。</p> <p>如果原字符串的长度，等于或大于最大长度，则字符串补全不生效，返回原字符串。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'xxx'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'xxx'</span>
<span class="token string">'xxx'</span><span class="token punctuation">.</span><span class="token function">padEnd</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token string">'ab'</span><span class="token punctuation">)</span> <span class="token comment">// 'xxx'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果用来补全的字符串与原字符串，两者的长度之和超过了最大长度，则会截去超出位数的补全字符串。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'abc'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'0123456789'</span><span class="token punctuation">)</span>
<span class="token comment">// '0123456abc'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果省略第二个参数，默认使用空格补全长度。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// '   x'</span>
<span class="token string">'x'</span><span class="token punctuation">.</span><span class="token function">padEnd</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token comment">// 'x   '</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>padStart()</code>的常见用途是为数值补全指定位数。下面代码生成 10 位的数值字符串。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'1'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'0'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;0000000001&quot;</span>
<span class="token string">'12'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'0'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;0000000012&quot;</span>
<span class="token string">'123456'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'0'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;0000123456&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>另一个用途是提示字符串格式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'12'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'YYYY-MM-DD'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;YYYY-MM-12&quot;</span>
<span class="token string">'09-12'</span><span class="token punctuation">.</span><span class="token function">padStart</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token string">'YYYY-MM-DD'</span><span class="token punctuation">)</span> <span class="token comment">// &quot;YYYY-09-12&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="实例方法-trimstart-修剪头部-trimend-修剪末尾"><a href="#实例方法-trimstart-修剪头部-trimend-修剪末尾" class="header-anchor">#</a> 实例方法：trimStart() 修剪头部，trimEnd() 修剪末尾</h2> <p><a href="https://github.com/tc39/proposal-string-left-right-trim" target="_blank" rel="noopener noreferrer">ES2019<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 对字符串实例新增了<code>trimStart()</code>和<code>trimEnd()</code>这两个方法。它们的行为与<code>trim()</code>一致，<code>trimStart()</code>消除字符串头部的空格，<code>trimEnd()</code>消除尾部的空格。它们返回的都是新字符串，不会修改原始字符串。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> s <span class="token operator">=</span> <span class="token string">'  abc  '</span><span class="token punctuation">;</span>

s<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc&quot;</span>
s<span class="token punctuation">.</span><span class="token function">trimStart</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;abc  &quot;</span>
s<span class="token punctuation">.</span><span class="token function">trimEnd</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// &quot;  abc&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>trimStart()</code>只消除头部的空格，保留尾部的空格。<code>trimEnd()</code>也是类似行为。</p> <p>除了空格键，这两个方法对字符串头部（或尾部）的 tab 键、换行符等不可见的空白符号也有效。</p> <p>浏览器还部署了额外的两个方法，<code>trimLeft()</code>是<code>trimStart()</code>的别名，<code>trimRight()</code>是<code>trimEnd()</code>的别名。</p> <h2 id="实例方法-matchall"><a href="#实例方法-matchall" class="header-anchor">#</a> 实例方法：matchAll()</h2> <p><code>matchAll()</code>方法返回一个正则表达式在当前字符串的所有匹配，详见《正则的扩展》的一章。</p></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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